The demands of modern communication standards on the signal quality of transmission devices or the signal quality of transmission devices in transceivers are rising with the growing need for high data rates and with increasing mobility. The now customary multistandard operating modes, for example UMTS, WLAN, require the use of bandwidth efficient, linear modulation types such as QPSK or QAM (Quadrature Amplitude Modulation). This results in a high linearity demand for a transmission device or for the transmission path in a transceiver. In this context, a power amplifier within the transmission path is particularly important as it amplifies the signal which is to be transmitted to achieve the required output power. Such an amplifier should have a high level of linearity, that is to say a linear gain in a wide output power range. At the same time, the power amplifier should have a high level of efficiency, particularly in mobile appliances, which are powered by means of a storage battery. A high level of efficiency, that is to say a high ratio of generated radio frequency output power to used battery power, normally exists only in a range in which the RF response of the amplifier has a nonlinear profile, however.
In mobile communication appliances today, therefore, power amplifiers are used which represent the best possible compromise between linearity of the power amplifier and power consumption, as a result of suitable circuitry. This can be achieved through suitable biasing or a suitable load impedance at the output of the amplifier, as described in the documents by G. L. Madonna et al.: “Investigations of Linearity Characteristics for Large-Emitter Area GaAS HBT Power Stages”, GAAS 2001 Conference, London 2001 and Iwai et al.: “High Efficiency and High Linearity InGaP/GaAS HBT Power Amplifiers: Matching Techniques of Source and Load Impedance to Improve Phase Distortion and Linearity”, IEEE Transactions on Electronic Devices, Vol. 45, No 6, June 1998. If the linearity of the power amplifier used within the transmission path needs to be improved further; it is customary practice in modern circuits to predistort the input signal for the power amplifier.
The predistortion of the input signal is in a form such that it compensates for the nonlinear gain of the amplifier. In this context, the baseband signal from the transmission device is predistorted at a suitable point. Examples of predistortion within the analog signal processing chain of the baseband are described in the documents Yamauchi et al.: “A Novel Series Diode Linearizer for Mobile Radio Power Amplifiers”, IEEE MTT S 1996, pages 831 to 833 and E. Westesson et al.: “A Complex Polynomial Predistorter Chip in CMOS for Baseband or IF Linearization of RF Power Amplifiers”, IEEE International Symposium on Circuits and Systems 1999. These analog predistortions can be provided inexpensively using simple supplementary circuits. A drawback of analog predistortion, however, is that the operating conditions, such as temperature, operating point or modulation of the power amplifier, may be altered only within very narrow limits. The limits can be extended by an analog predistortion circuit which can be adapted flexibly. However, such predistortion circuits can be produced only with a high level of complexity and also increase the power consumption again.
In contrast to this, predistortion of the digital baseband signal is very well matched to changing external operating conditions. In the case of adaptive digital predistortion, a portion of the analog output signal behind the power amplifier is output, demodulated and converted into a digital baseband signal again. From the comparison between the converted baseband signal and the original, undistorted baseband signal it is possible to determine the distortion in a portion of the transmission path and particularly in the power amplifier. From this, predistortion coefficients can be calculated. An embodiment with adaptive predistortion is shown in document US 2003/0035494. The drawback of the arrangement presented therein, however, is the high power consumption on account of the continuously operating predistortion unit.